The "flu jab" is recommended only for people at high risk (various chronic diseases, age over 65, NHS workers, carers). Influenza and colds are caused by viruses and we all tend to get them in winter because we're in more contact, which allows them to spread more easily. In the tropics there are more in the wet season for the same reason.
I can't comment on individual cases but there's no particular reason why colds or flu should be worse in otherwise healthy immigrants"Man flu" is something else and off topic !
